allow full-range file offsets to mmap on archs with 64-bit syscall args

normally 32-bit archs use the mmap2 syscall and are limited to an
offset of 2^32 pages. however some 32-bit archs (mainly ILP32-on-64
ones like x32) have 64-bit syscall argument slots and thus can accept
the full range. don't artifically limit them.

src/mman/mmap.c

@@ -10,7 +10,7 @@ static void dummy(void) { }
 weak_alias(dummy, __vm_wait);
 
 #define UNIT SYSCALL_MMAP2_UNIT
-#define OFF_MASK ((-0x2000ULL << (8*sizeof(long)-1)) | (UNIT-1))
+#define OFF_MASK ((-0x2000ULL << (8*sizeof(syscall_arg_t)-1)) | (UNIT-1))
 
 void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off)
 {